This invention relates to a method and apparatus for calibrating rapid sequence radiography, and particularly, though not limited to, cinematic angiograms.
A need exists for an objective method for accurate quantitation of in vivo coronary arterial stenoses. Such a method not only would be useful in the clinical management of individual patients, but also could demonstrate any potential effect of an intervention in attempting to treat arteriosclerotic coronary artery disease in general.
Single plane coronary cinematic angiograms provide potentially useful information about the luminal dimensions of a coronary artery. However, visual inspection is inadequate in measuring "percent" coronary arterial stenoses from coronary cinematic angiograms because of problems of reproducibility and accuracy. As a result, objective methods have been developed in order to more accurately determine the vessel shadow edge, so that a measure of vessel lumen width as well as vessel edge regularity can be obtained. However, no equally accurate method for determining lumen depth is available.
Although the thickness of contrast material at any point within a coronary artery is a function of depth, and the light intensity through a corresonding point of a radiograph is a function of contrast material thickness, the latter relationship is complex and varies with multiple factors, including characteristics of the x-ray beam, film speed and development, and radio-density distribution of objects in the fluoroscopic field; in addition, there is a spontaneous variation in the relationship with time despite constancy of all other factors. What is required is a standard for objective calibration of film gray scale against contrast material thickness.
Materials fabricated in the shape of wedges or stepped wedges have been used in radiology for multiple purposes, including standardizing exposures and providing a reference of comparison for densitometric measurement of object thickness, object density, etc. Typical of such conventional radiographic standards are disclosed in U.S. Pat. Nos. 2,399,650 to Mayer for determining the thickness of hollow aircraft propellar blades and 3,088,027 to Graham for radiographic examination of tubing. See also U.S. Pat. Nos. 1,953,249 to Michel and 2,426,884 to Kieffer. Such radiographic standards have necessarily been limited to situations wherein the background, over which a wedge is superimposed, is homogenous in thickness across the object plane, so that x-ray intensity through the background is the same for each step of the wedge. However, in clinical situations, variability in tissue thickness across the plane perpendicular to the direction of the x-ray beam usually results in a nonuniform radiographic field over the wedge.
Because of the focal nature of the atherosclerotic process, nonaxisymmetric reductions in luminal cross sections are common. Visual inspection of coronary angiograms is limited primarily to luminal edge detection, and, as a result, enface plaques cannot be quantitated from a single radiographic view. Moreover, even for cylindrical lumens, the error of a derived cross-sectional area estimate is a squared function of the error in diameter measurement.
Utilization of all the image information between luminal edges should increase the accuracy of cross-sectional area estimates beyond that derived from edge identification alone. In addition, successful extraction of such three-dimensional information should yield a rotationally invariant measure of cross-sectional area. However, when the radio-density of an entire radiographic field is inhomogeneous, it is not possible to calibrate film gray scale against object thickness in a conventional manner by incorporation of radiographic standards within the field. Crawford, et al., circumvented this problem by modifying an empirically derived mathematical expression characterizing the photographic response of film as described by S. K. Hilal, "Determination of the blood flow by a radiographic technique, Physical considerations and experimental results," Amer J Roentgen 96: 896, 1966., so that relative arterial cord lengths within a femoral arterial cross section could be calculated accurately from knowledge of film optical densities at zero and saturation exposures as well as over the luminal image and its adjacent background. However, radiographic and film development conditions had to be carefully controlled, and the technique has not been applied to radiographic systems employing an image intensifier.
From the foregoing it is evident that the primary problem which has prevented objective calibration of film gray scale against contrast material quantity, a prerequisite to cross-sectional area determination, is the inhomogeneity of the radiographic field during patient exposure. Consequently, conventional radiographic standards of the type referred to above have not been used. Moreover automatic exposure settings, as commonly used, preclude exposing filming standards at times other than during patient exposure.